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| United States Patent |
6,180,533
|
|
Jain
, et al.
|
January 30, 2001
|
Method for etching a trench having rounded top corners in a silicon
substrate
Abstract
The present disclosure includes a method of plasma etching a trench having
rounded top corners in a silicon substrate. One embodiment includes the
following general steps: a) providing a semiconductor structure comprising
a hard masking layer, overlying a silicon substrate; b) plasma etching
through said hard masking layer and any additional underlying layers
overlying said silicon substrate using at least one plasma feed gas which
does not provide polymer deposition on surfaces of said semiconductor
structure during etching; where said plasma etching exposes a face of said
silicon substrate; and c) plasma etching at least a first portion of a
trench into said silicon substrate using reactive species generated from a
feed gas comprising a source of fluorine, a source of carbon, a source of
hydrogen, and a source of high energy species which provide physical
bombardment of said silicon substrate. Top corner rounding is effected by
deposition of a thin layer of polymer on a top corner of the trench during
etching of the first portion of the trench, resulting in the formation of
a rounded "shoulder" at the top corner of the trench. Typically a layer of
silicon oxide overlies at least a portion of the silicon substrate
surface. The method described provides excellent critical dimension
control over the active area of a transistor produced using the method and
reduces the need to remove polymer from substrate and reactor surfaces
after etching of the silicon trench.
| Inventors:
|
Jain; Alok (Singapore, SG);
Low; Michelle Siew Mooi (Singapore, SG);
Zou; Gang (Singapore, SG);
Mui; David (Santa Clara, CA);
Podlesnik; Dragan (Palo Alto, CA);
Liu; Wei (Sunnyvale, CA)
|
| Assignee:
|
Applied Materials, Inc. (Santa Clara, CA)
|
| Appl. No.:
|
545700 |
| Filed:
|
April 7, 2000 |
| Current U.S. Class: |
438/714; 257/E21.218; 257/E21.232; 257/E21.549; 438/719; 438/724; 438/739 |
| Intern'l Class: |
H01L 021/00 |
| Field of Search: |
438/713,714,719,724,737,739,744
216/2,67,79
|
References Cited
U.S. Patent Documents
| 4729815 | Mar., 1988 | Leung | 438/719.
|
| 4857477 | Aug., 1989 | Kanamori | 438/719.
|
| 5843846 | Dec., 1998 | Nguyen et al. | 438/713.
|
Other References
C. P. Chang et al., IEDM Tech. Digest, p. 661 (1997).
A. Chatterjee et al., IEDM Tech. Digest, p. 829 (1996).
H. Watanabe et al., IEDM Tech. Digest, p. 833 (1996).
|
Primary Examiner: Powell; William
Attorney, Agent or Firm: Church; Shirley L.
Parent Case Text
RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser. No.
09/371,966, filed Aug. 10, 1999.
Claims
We claim:
1. A method of plasma etching a trench having a rounded top corner in a
silicon substrate, the method comprising the steps of:
a) providing a semiconductor structure comprising a hard masking layer,
overlying a silicon substrate;
b) plasma etching through said hard masking layer and any additional
underlying layers overlying said silicon substrate using at least one
plasma feed gas which does not provide polymer deposition on surfaces of
said semiconductor structure during etching, where said etching exposes a
surface of said silicon substrate; and
c) plasma etching at least a first portion of a trench into said silicon
substrate using reactive species generated from a feed gas comprising a
source of fluorine, a source of carbon, a source of hydrogen, and a source
of high energy species which provide physical bombardment of said silicon
substrate.
2. The method of claim 1, wherein said hard mask comprises silicon nitride.
3. The method of claim 2, wherein a layer of silicon oxide overlies at
least a portion of said silicon substrate surface.
4. The method of claim 1, or claim 2, or claim 3, wherein said hard mask is
plasma etched using a feed gas comprising SF.sub.6 or CF.sub.4 or a
combination thereof, which is used in combination with HBr.
5. The method of claim 3, wherein said silicon oxide layer is plasma etched
using a feed gas which does not contain hydrogen.
6. The method of claim 5, wherein said silicon oxide layer is plasma etched
using a feed gas which comprises CF.sub.4 and argon.
7. The method of claim 4, wherein said source of fluorine is selected from
the group consisting of CF.sub.4, CHF.sub.3, CH.sub.2 F.sub.2, and
combinations thereof.
8. The method of claim 4, wherein said source of carbon is selected from
the group consisting of CF.sub.4, CHF.sub.3, CH.sub.2 F.sub.2, and
combinations thereof.
9. The method of claim 4, wherein said source of hydrogen is selected from
the group consisting of HBr, CHF.sub.3, CH.sub.2 F.sub.2, and combinations
thereof.
10. The method of claim 1, wherein said source of high energy species is
selected from the group consisting of HBr, argon, krypton, xenon, helium,
and combinations thereof.
11. The method of claim 1, wherein step c) is performed using a feed gas
comprising CHF.sub.3, HBr, and CF.sub.4.
12. The method of claim 4, wherein step c) is performed using a feed gas
comprising CHF.sub.3, HBr, and CF.sub.4.
13. The method of claim 1, wherein step c) is performed using a feed gas
comprising CHF.sub.3, argon, and CF.sub.4.
14. The method of claim 4, wherein step c) is performed using a feed gas
comprising CHF.sub.3, argon, and CF.sub.4.
15. The method of claim 1, wherein step c) is performed using a feed gas
comprising CH.sub.2 F.sub.2, HBr, and CF.sub.4.
16. The method of claim 4, wherein step c) is performed using a feed gas
comprising CH.sub.2 F.sub.2, HBr, and CF.sub.4.
17. The method of claim 1, wherein step c) is performed using a feed gas
comprising CH.sub.2 F.sub.2, argon, and CF.sub.4.
18. The method of claim 4, wherein step c) is performed using a feed gas
comprising CH.sub.2 F.sub.2, argon, and CF.sub.4.
19. The method of claim 1, wherein step c) is performed using a feed gas
which does not contain nitrogen.
20. The method of claim 4, wherein step c) is performed using a feed gas
which does not contain nitrogen.
21. The method of claim 1, wherein step c) is performed using a process
chamber pressure of at least 40 mTorr.
22. The method of claim 21, wherein step c) is performed using a process
chamber pressure within the range of about 40 mTorr to about 90 mTorr.
23. The method of claim 1, wherein step c) is performed for a time period
within the range of about 10 seconds to about 30 seconds.
24. The method of claim 1 wherein etching of said silicon substrate in step
c) is performed to a depth of about 50 .ANG. to about 200 .ANG..
25. The method of claim 1, wherein after the completion of step c), said
feed gas used for etching a remainder of said trench is adjusted in
composition, to reduce polymer deposition during etching of said remainder
of said trench.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method for etching a trench in a
silicon substrate. In particular, the present invention pertains to
particular plasma etch chemistries, process conditions, and a series of
steps which may be used to produce rounded top corners on a trench etched
in a silicon substrate. The term "top trench corner" refers to the
transition from the upper sidewall of a trench to the upper surface of the
substrate in which the trench is formed.
2. Brief Description of the Background Art
Trenches formed in silicon using traditional etching methods typically have
sharp, squared-off top corners. These sharp, squared-off corners lead to
high field stress in film layers subsequently deposited thereon during
further processing steps. The high field stress can potentially lead to
the electrical breakdown of the overlying deposited film layers. Further,
the sharp, squared-off corners are a point of charge accumulation, which
can cause edge leakage. Rounding of the top corners in trench structures
can be critical for device performance. However, rounding of the corners
in a manner which results in a loss of device active area is undesirable.
Various methods for obtaining a rounded top corner on a trench formed in a
silicon substrate are known in the art. For example, U.S. Pat. No.
5,843,846, issued Dec. 1, 1998 to Nguyen et al., discloses a method for
rounding the top corners of a sub-micron trench in a semiconductor device
after trench formation. The method comprises exposing the previously
formed trench to a gas comprising a carbon-fluorine gas, argon, and
nitrogen directly after trench formation. The combination of the
carbon-fluorine and nitrogen gases etch back the silicon nitride and
stress relief oxide layers in order to expose the top corners of the
trench. As the top corners of the substrate are exposed, nitrogen and
argon gases are said to sputter the top corners, rounding them as the etch
process completes the trench. The method is preferably performed using a
low density parallel plate etch reactor.
Commonly assigned, copending U.S. application Ser. No. 09/042,249, filed
Mar. 13, 1998, discloses a method of obtaining a rounded top corner on a
trench formed in a semiconductor substrate comprising the following steps:
(a) providing a film stack comprising the following layers, from the upper
surface of the film stack toward the underlying substrate, (I) a first
layer of patterned material (typically, a patterned photoresist) which is
resistant to a wet etch solution used to etch an underlying second layer
and which is resistant to dry etch components used to etch the
semiconductor substrate (typically, silicon), and (ii) a second layer of
material (typically, silicon dioxide) which can be preferentially etched
using a wet etch solution, wherein the second layer of material is
deposited directly on top of the semiconductor substrate; (b) wet etching
the second layer by immersing the film stack in a wet etch solution for a
period of time sufficient to form an undercut beneath the first layer and
to expose the underlying semiconductor substrate; and c) isotropically dry
etching the exposed semiconductor substrate so as to form a trench in the
semiconductor substrate.
A sharp corner at the bottom of a trench can also be a source of stress,
causing problems of the kind described with reference to the top corners
of the trench. In addition, a rounded corner facilitates filling of the
trench with a reduced possibility of trapping voids within the fill
material. It is desirable to provide an etch process which provides a
rounded bottom corner while maintaining a desired trench sidewall angle,
for example, about 80.degree. to 90.degree..
SUMMARY OF THE INVENTION
We have discovered an improved method of obtaining rounded top trench
corners on a trench formed in a silicon substrate. In particular, a hard
mask (for example and not by way of limitation, silicon nitride) and
typically an underlying silicon oxide layer are first pattern etched using
"clean" etch chemistries; that is, etch chemistries that do not result in
the deposition of polymer on feature surfaces during etching. After
completely etching through the hard mask and a silicon oxide layer, when
present, a top corner rounding step is performed using etch chemistry
which results in the deposition of a thin layer of polymer on the upper
edge (top corner) of the silicon trench. The thin layer of polymer forms a
rounded "shoulder" at the top corner of the trench. Thus, top corner
rounding is effected during the etching of the first portion of the
silicon trench.
The use of "clean" chemistries to etch overlying layers such as a silicon
nitride or silicon oxide layer minimizes the total amount of polymer
generated during the entire etch process, and reduces polymer removal
problems after etch. Because the polymer-generating corner rounding step
is performed after etching of an overlying layer of silicon oxide in many
cases, we are able to define the device active area critical dimension
during the oxide etch step. We are able to maintain a stable and
reproducible active area critical dimension control.
In one embodiment, the method comprises the following steps: a) providing a
semiconductor structure comprising a hard masking layer, overlying a
silicon substrate; b) plasma etching through the hard masking layer and
any additional underlying layers overlying the silicon substrate using at
least one plasma feed gas which does not provide polymer deposition on
surfaces of the semiconductor structure during etching, where the etching
exposes a surface of the silicon substrate; and c) plasma etching at least
a first portion of a trench into the silicon substrate using reactive
species generated from a feed gas comprising a source of fluorine, a
source of carbon, a source of hydrogen, and a source of high energy
species which provide physical bombardment of the silicon substrate.
Plasma feed gas recipes which provide excellent results in the top corner
rounding step include CHF.sub.3 /HBr/CF.sub.4 or CHF.sub.3
/argon/CF.sub.4. Less preferably,CH.sub.2 F.sub.2 may be used in place of
CHF.sub.3. Use of CH.sub.2 F.sub.2 may result in increased polymer
deposition, and require the adjustment of the relative amounts of the
gaseous components making up the plasma feed gas.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an individual polysilicon etch chamber of the type
used in an Applied Materials' CENTURA.RTM. DPS.TM. polysilicon etch
system, which is an example of an etch processing apparatus which can be
used for performing the method of the invention.
FIG. 2 is a schematic of a cross-sectional side view of an individual
CENTURA.RTM. DPS.TM. polysilicon etch chamber 102.
FIG. 3 is a schematic of an Applied Materials' MXP+ polysilicon etch
chamber, which is an example of an etch processing apparatus which can be
used for performing the method of the invention.
FIG. 4A shows a typical beginning structure 400 for practicing a first top
corner rounding embodiment method of the invention. This structure 400
comprises, from top to bottom, a patterned photoresist layer 408, a layer
of silicon nitride 406, a layer of silicon oxide 404, and an underlying
silicon substrate 402.
FIG. 4B shows the structure 400 of FIG. 4A after pattern etching of the
silicon nitride layer 406. The top surface 410 of the silicon oxide layer
404 is exposed during the silicon nitride pattern etching step.
FIG. 4C shows the structure 400 of FIG. 4B after the break-through step in
which the silicon oxide layer 404 and the top portion of the silicon
substrate 402 are etched. The top corners 412 of the silicon substrate 402
are rounded during this silicon oxide breakthrough step.
FIG. 4D shows the structure 400 of FIG. 4C after etching of a shallow
trench 416 into the silicon substrate 402.
FIG. 5A shows a typical beginning structure 500 for practicing a second top
corner rounding embodiment method of the invention. This structure 500
comprises, from top to bottom, a patterned silicon nitride hard mask 506,
an adhesion layer of silicon oxide 504, and an underlying silicon
substrate 502.
FIG. 5B shows the structure 500 after a break-through step in which the
pattern is transferred through silicon oxide adhesion layer 504 and a
built-up structure 508 is created on the side wall 507 of silicon nitride
hard mask 506.
FIG. 5C shows the structure 500 after a first silicon trench etch step in
which a shallow trench 518 is etched into silicon substrate 502 to a depth
"A" which is equal to about 85-90% of the final desired depth for trench
518.
FIG. 5D shows the structure 500 after a second etch step in which the
shallow trench 518 is etched to its desired final depth B into silicon
substrate 502. The bottom corners 520 of the trench 518 are rounded during
this second etch step.
FIGS. 6A through 6F show a series of schematics of trench wall shapes which
can be obtained using the method of the invention, where the radius of the
trench corners at the top and bottom of the trench, as well as the
sidewall slope are controlled.
FIG. 7 shows a computer adapted to control a plasma etch system, to carry
out the method of the invention.
FIG. 8A shows a typical beginning structure 800 for practicing a third top
corner rounding embodiment method of the invention. This structure 800
comprises, from top to bottom, a patterned photoresist layer 808, a layer
of silicon nitride 806, a layer of silicon oxide 804, and an underlying
silicon substrate 802.
FIG. 8B shows the structure 800 of FIG. 8A after pattern etching of the
silicon nitride layer 806.
FIG. 8C shows the structure 800 of FIG. 8B after plasma etching the silicon
oxide layer 804. The silicon oxide layer 804 has been completely etched,
so that the top surface 811 of the silicon substrate 802 is exposed. FIG.
8C shows no over etch after etch patterning of silicon oxide layer 804. In
some instances, an overetch (not shown) may be helpful in providing better
critical dimension control.
FIG. 8D shows the structure 800 of FIG. 8C after the top corner rounding
step.
FIG. 8E shows the structure 800 of FIG. 8D after etching of a shallow
trench 816 into the silicon substrate 802.
FIGS. 9A through 9C show schematics of a structure 900 after a top corner
rounding step.
FIG. 9A shows a structure 900 comprising, from top to bottom, a patterned
masking layer of silicon nitride 906, an etched layer of silicon oxide
904, and an underlying silicon substrate 902.
FIG. 9B shows an enlargement of the rounded corner achieved using an
embodiment of the method of the present invention, where the silicon oxide
layer 904 is not overetched into the silicon substrate prior to the top
corner rounding step.
FIG. 9C shows an enlargement of the rounded corner achieved using an
embodiment of the method where the silicon oxide layer is overetched into
the silicon substrate prior to the top corner rounding step, to provide a
gauged top rounded corner.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to corner rounding during the etching of
silicon trenches. Disclosed herein are three methods for top corner
rounding and a method for bottom corner rounding which can be used in
combination with any of the three top corner rounding methods.
In the embodiments described below, a typical etch stack used to form a
trench in a silicon substrate is, from top to bottom, a patterned
photoresist, a layer of silicon nitride, and a layer of silicon oxide, all
overlying a silicon substrate into which a trench is to be etched.
A first method of creating a rounded top corner on an etched silicon trench
comprises etching both the silicon oxide layer and an upper portion of the
underlying silicon substrate during a "break-through" step which
immediately precedes the step in which the silicon trench is etched. The
break-through step is used for removal of the silicon oxide layer
overlying silicon substrate areas in which a trench is to be etched. The
plasma feed gas for the break-through step comprises carbon and fluorine.
The atomic ratio of fluorine to bromine in the plasma feed gas is
preferably within the range of about 10:1 to about 50:1.
In this method, the photoresist layer is preferably not removed prior to
the break-through etching step. Subsequent to the break-through step, a
trench is etched to a desired depth in the silicon substrate using a
different plasma feed gas composition, which does not provide polymer
formation. Both the silicon oxide etch and silicon trench etch may be
performed in a single processing chamber, reducing processing time,
providing increased throughput, and providing a decrease in processing
costs. In some instances, other etch stack layers, such as the photoresist
layer and the silicon nitride layer, may be removed in the same process
chamber, using a plasma which produces byproducts compatible with the
plasmas used in subsequent device processing steps. The principal etchant
of the break-through etch plasma is generated from a feed gas containing
fluorine which may be, by way of example and not by way of limitation,
selected from the group consisting of CF.sub.4, CHF.sub.3, CH.sub.2
F.sub.2, CH.sub.3 F, and combinations thereof. The principal etchant is
preferably selected from the group consisting of CF.sub.4, CHF.sub.3, and
combinations thereof; and CF.sub.4 has been shown to work very well. The
plasma feed gas may further include CH.sub.4. In some instances, the
presence of CH.sub.4 may be required, for example, when the amount of
photoresist material available during the silicon oxide layer etch is
minimal. The plasma feed gas preferably further includes a nonreactive,
diluent gas selected from the group consisting of argon, helium, xenon,
krypton, and combinations thereof. The nonreactive, diluent gas is most
preferably argon. The plasma feed gas may further include a controlled
amount of oxygen, which may be used to improve the top corner rounding
effect. The radius of the rounded corner depends on the feature size of
the structure being etched. For a feature size of about 0.35 .mu.m, a
typical radius for a rounded top corner ranges from about 15 nm to about
25 nm.
A second method of creating a rounded top corner on the etched silicon
trench comprises formation of a built-up extension on the sidewall of the
patterned silicon nitride layer during etch (break-through) of the
underlying silicon oxide adhesion layer. The built-up extension on the
silicon nitride sidewall acts as a sacrificial masking material during
etching of the silicon trench, delaying etching of the silicon at the
outer edges of the top of the trench. This permits completion of trench
etching with delayed etching of the top corner of the trench, and provides
a more gentle rounding (increased radius) at the top corners of the
trench. For example, for a feature size of about 0.16 .mu.m, a typical
radius for a rounded top corner ranges from about 25 nm to about 40 nm.
In this second method of creating a rounded top corner, the photoresist
used to pattern the silicon nitride hard mask is removed prior to the
silicon oxide break-through step. The plasma feed gas used during the
break-through step must provide a source of hydrogen, a source of
fluorine, a source of carbon, and a source of surface bombardment atoms.
Typically, the surface bombardment atom source is an inert plasma feed gas
such as argon, helium, krypton, nitrogen, xenon, or a combination thereof.
Argon works particularly well. A single compound may be used to provide
the hydrogen, carbon, and fluorine. Typically, more than one gaseous
compound is used. It is important to have a controlled amount of carbon
present in the process chamber at the time of the break-through step.
While the hydrogen and the carbon are used to form polymeric material
which deposits upon the silicon nitride sidewall, the fluorine and the
bombardment atoms (typically argon) provide for removal of native oxide
layers. Examples of hydrogen sources other than hydrogen gas include HBr
(which requires an increased amount of carbon in the process chamber to
work well), CHF.sub.3, CH.sub.2 F.sub.2, CH.sub.3 F, NH.sub.3, CH.sub.4,
and combinations thereof. Several of these hydrogen-containing compounds
may also serve as carbon and fluorine sources. An excellent source of
carbon and fluorine is CF.sub.4. One of the plasma feed gas recipes which
provides excellent results is a combination of CF.sub.4, HBr, and argon.
During the etching of the silicon trench which follows the break-through
step, it is desirable to round the bottom corners of the finished silicon
trench. We have developed a method for rounding the bottom corners which
may be used in combination with either of the top corner rounding methods
described above. The plasma used to etch the silicon trench is one
commonly used in the art and is different from that used for the top
corner rounding methods. In some instances, the substrate may be removed
from the processing chamber prior to the silicon etch, but preferably the
silicon oxide etch and silicon trench etch are performed in sequence in a
single processing chamber.
In at third top corner rounding method, the silicon nitride hard mask and
silicon oxide layer are first etched using "clean" etch chemistries; that
is, etch chemistries that do not result in the deposition of polymer on
feature surfaces during etching. After completely etching through the
silicon oxide layer (and in some instances overetching slightly into the
silicon substrate), a separate top corner rounding step is performed using
etch chemistry which results in the deposition of a thin layer of polymer
on a top corner of the trench, resulting in the formation of a rounded
"shoulder" at the top corner of the trench. In particular, the top corner
rounding is effected during the etching of the first portion of the
silicon trench (not during the silicon oxide etch step). This reduces the
amount of polymer which must be stripped after completion of etching of
the trench, while providing excellent critical dimension control in a
transistor device active area.
Typically, the hard mask comprises an inorganic material such as silicon
nitride, which is plasma etched using a feed gas which does not result in
polymer deposition on surfaces of the semiconductor structure during
etching. The hard mask is preferably plasma etched using a feed gas which
does not contain carbon. The feed gas used to etch the silicon nitride
layer typically comprises SF.sub.6 or CF.sub.4, or a combination thereof,
which is used in combination with HBr.
When there is a silicon oxide layer underlying the hard masking layer, the
silicon oxide layer is typically plasma etched using a feed gas which does
not contain hydrogen. Plasma etching of the silicon oxide layer is
typically performed using a feed gas comprising CF.sub.4 and argon.
After etching of the layer directly overlying the silicon substrate
(typically a silicon oxide layer) a top corner rounding step is performed.
The corner rounding step is performed during etch of a first portion of a
trench into the underlying silicon substrate. The etching is carried out
using reactive species generated from a feed gas comprising a source of
fluorine, a source of carbon, a source of hydrogen, and a source of high
energy species which provides physical bombardment of the silicon
substrate. A single compound (e.g., CHF.sub.3,) may be used to provide the
hydrogen, carbon, and fluorine. Typically, more than one gaseous compound
is used. The source of fluorine is typically selected from the group
consisting of CF.sub.4, CHF.sub.3, CH.sub.2 F.sub.2, and combinations
thereof The source of carbon is typically selected from the group
consisting of CF.sub.4, CHF.sub.3, CH.sub.2 F.sub.2, and combinations
thereof. The source of hydrogen is typically selected from the group
consisting of HBr, CHF.sub.3, CH.sub.2 F.sub.2, and combinations thereof.
During the top corner rounding step, the carbon and hydrogen in the feed
gas combine to provide deposition of a thin layer of polymer on a top
corner of the trench, resulting in the formation of a rounded "shoulder"
at the top corner of the trench. It is important to have a controlled
amount of carbon present in the process chamber at the time of the top
corner rounding step.
While the hydrogen and the carbon are used to form polymeric material which
deposits upon the top corner of the trench during etching of a first
portion of the trench, reactive fluorine-comprising species and high
energy physical bombardment species provide for removal of the silicon
trench substrate. The source of high energy physical bombardment species
is typically selected from the group consisting of HBr, argon, krypton,
xenon, helium, and combinations thereof.
Preferably, the top corner rounding step is performed using a process
chamber pressure of at least 40 mTorr, most preferably, within the range
of about 40 mTorr to about 90 mTorr. Depending on the desired final trench
depth, the top corner rounding step is typically performed for a time
period within the range of about 10 seconds to about 30 seconds. Etching
of the silicon substrate during the top corner rounding step is typically
performed to a depth of about 50 .di-elect cons. to about 200 .ANG..
After performance of the top corner rounding step, the remaining portion of
the trench is etched to a desired final depth, preferably using a feed gas
which does not contain carbon. Trench etching is typically performed using
conventional silicon etch chemistry, such as HBr/Cl.sub.2 /O.sub.2 or
Cl.sub.2 /O.sub.2 or HBr, or HBr/Cl.sub.2.
In most applications, a trench having rounded bottom corners is desired.
The term "bottom trench corner" refers to the transition from the lower
sidewall of a trench to the base (bottom) of the trench. The trench is
typically etched to a desired final depth in the silicon substrate using
conventional silicon etch chemistry, where the plasma feed gas comprises
chlorine, or oxygen, or a combination thereof. The plasma feed gas may
include SF.sub.6. We have discovered that a more rounded bottom trench
corner is obtained using a two-step silicon etch process. In the first
silicon etch step, the trench is etched to a depth within the range of
about 75% and about 95%, most preferably, within the range of about 80%
and 90%, of its desired final depth. The etch chamber pressure is within
the range of about 15 mTorr and about 40 mTorr. Preferably the chamber
pressure is about 20 to 30 mTorr. The etch plasma is a high density plasma
having an electron density ranging from about 10.sup.9 to 10.sup.12
e.sup.- cm.sup.3. In the second silicon etch step, the process chamber
pressure is increased to range between about 40 mTorr and about 90 mTorr.
Preferably, the process chamber pressure is between about 50 mTorr to
about 60 mTorr. In addition to a higher process pressure, a high substrate
bias is employed during the second etch step, to ensure that the etchant
species will be directed to the bottom of the trench, resulting in
rounding of the bottom trench corners Optionally, SF.sub.6 may be added to
the silicon trench etch plasma feed gas to assist in the corner rounding
process. For a feature size of about 0.16 .mu.m, a typical radius obtained
for a rounded bottom corner ranges from about 15 nm to about 25 nm. The
second step etch is carried out using the same etch chemistry, but at a
process chamber pressure within the range of about 40 mTorr to about 90
mTorr.
The detailed methods, preferred process parameters, and experimental
examples are provided below. As a preface to a detailed description of
preferred embodiments, it should be noted that, as used in this
specification and the appended claims, the singular forms "a", "an", and
"the" include plural referents, unless the context clearly dictates
otherwise.
I. AN APPARATUS FOR PRACTICING THE INVENTION
The method of the invention is preferably performed in an etch processing
apparatus which provides for separate power control of a plasma generation
source and a substrate biasing device. An example of such an apparatus is
the Applied Materials' CENTURA.RTM. polysilicon etch system. FIGS. 1 and 2
are schematics of an individual CENTURA.RTM. DPS.TM. polysilicon etch
chamber 102 of the type used in the Applied Materials' CENTURA.RTM.
polysilicon etch system. The CENTURA.RTM. DPS.TM. polysilicon etch chamber
102 is configured to be mounted on a standard CENTURA.RTM. 5200 etch
mainframe.
FIG. 1 shows a detailed schematic of an individual CENTURA.RTM. DPS.TM.
polysilicon etch chamber 102 of the type used in the CENTURA.RTM. etch
system. The CENTURA.RTM. DPS.TM. polysilicon etch chamber 102 consists of
an upper chamber 104 having a ceramic dome 106, and a lower chamber 108.
The lower chamber 108 includes a monopolar electrostatic chuck (ESC)
cathode 110. Gas is introduced into the chamber via four ceramic gas
injection nozzles 114. Chamber pressure is controlled by a closed-loop
pressure control system 118 with a throttle valve 116.
FIG. 2 shows a schematic of a cross-sectional side view of the polysilicon
etch chamber 102. During processing, a substrate 220 is introduced into
the lower chamber 108 through inlet 222. The substrate 220 is held in
place by means of a static charge generated on the surface of
electrostatic chuck (ESC) cathode 110 by applying a DC voltage to a
conductive layer located under a dielectric film on the chuck surface. The
cathode 110 and substrate 220 are then raised by means of a wafer lift 224
and sealed against the upper chamber 104 in position for processing. Etch
gases are introduced into the upper chamber 104 via the ceramic gas
injection nozzles 114. The polysilicon etch chamber 102 uses an
inductively coupled source 226 at 12.56 MHZ for generating and sustaining
a high density plasma. The wafer is biased with an RF source 230 at 13.56
MHZ. Power to the plasma source 226 and substrate biasing means 230 are
controlled by separate controllers, 228 and 232, respectively.
An endpoint subsystem (not shown) senses the end of the etch process by
monitoring changes in the light emitted by the plasma in the etch chamber
102. The standard CENTURA.RTM. DPS.TM. endpoint system consists of
amonochromator and photomultiplier tube which automatically endpoints all
etch chambers. A fiber optic cable routes light from a recessed quartz
window on the chamber to the monochromator or an optional HOT (High
Optical Throughput) pack photomultiplier. When the monochromator is used,
light is shone into a motor-driven concave grating. Light is then
reflected onto the entrance slit on the photomultiplier tube, which
amplifies the light. This data is then displayed on a PC monitor. The
operator sets an algorithm which controls the endpoint system. Overetching
can be programmed to start either as the film begins to clear or when it
has cleared completely. The endpoint time can be adjusted by changing the
number of windows that the signal must exit to endpoint. The endpoint
wavelength is programmable for each process step. An appropriate endpoint
wavelength is selected depending on the films being etched.
Alternatively, the method of the invention may be performed in an etch
processing apparatus wherein power to a plasma generation source and power
to a substrate biasing means are controlled by a single power control,
such as the Applied Materials' MXP or MXP+polysilicon etch chamber. FIG. 3
is a schematic of an Applied Materials' MXP+polysilicon etch chamber 300,
which is a parallel plate plasma etch chamber of the kind which is
well-known in the art. The MXP+polysilicon etch chamber offers advantages
over other similar etch chambers in that it includes a simplified,
two-dimensional gas distribution plate 302, which allows for more uniform
gas distribution throughout the chamber. Another modification is a
removable aluminum chamber liner 304, which can be easily removed and
replaced during each wet cleaning procedure, allowing for a more rapid
cleaning cycle. Yet another modification is an improved focus ring 306,
which moves together with (rather than independently from) the cathode
308, resulting in reduced particle generation due to fewer moving parts
within the apparatus. The high temperature cathode 308 has independent
temperature control (not shown), which functions in response to a
temperature reading from pedestal temperature probe 312, which permits
operation at a temperature in excess of the process chamber temperature.
The substrate to be processed (not shown) rests on an electrostatic chuck
pedestal 310, which is joined to cathode 308.
As would be appreciated by one skilled in the art, the inventive method may
be used in a variety of etch chambers, such as those described in, for
example, U.S. Pat. Nos. 5,242,538; 5,705,081; 5,534,751; and 5,698,062.
II. A FIRST METHOD OF ETCHING A SILICON TRENCH HAVING ROUNDED TOP CORNERS
FIG. 4A shows a schematic of a semiconductor structure 400 comprising an
etch stack overlying a silicon substrate 402. This etch stack comprises,
from top to bottom, a patterned photoresist layer 408, a silicon nitride
layer 406, and a silicon oxide layer 404. The thickness of the patterned
photoresist layer 408 is typically within the range of about 4000 .ANG. to
about 10,000 .ANG.. The silicon nitride layer 406 typically has a
thickness within the range of about 1000 .ANG. to about 2000 .ANG.. The
silicon oxide layer 404 typically has a thickness within the range of
about 50 .ANG. to about 350 .ANG.. The relative thicknesses of the film
layers illustrated in FIGS. 4A-4D are not to scale. The drawings are
intended only to show the relative positions of the various layers on
silicon substrate 402.
The patterned photoresist layer 408 may be any suitable photoresist
material patterned using techniques known in the art. Typically, the
photoresist layer material is an organic, carbon-containing material. The
thickness and patterning method for the photoresist 408 will depend on the
particular photoresist material used. A frequently used photoresist is a
DUV photoresist available from either JSR or Shipley. A typical film
thickness for a DUV photoresist ranges from about 4,000 to about 10,000
.ANG..
Referring to FIG. 4B, the silicon nitride layer 406 has been pattern etched
(opened) to expose a surface 410 of the silicon oxide layer 404.
Preferably, the silicon nitride layer 406 is pattern etched using a plasma
generated from a plasma feed gas comprising SF.sub.6, in combination with
a profile control additive such as HBr or CHF.sub.3. Etch process
conditions are apparatus-sensitive, but one skilled in the art can
determine appropriate conditions for a given apparatus by minimal
experimentation, based on process conditions generally known in the art.
A "break-through" step is then performed to remove the silicon oxide layer
404 from the open patterned areas overlying silicon substrate 402. As
shown in FIG. 4C, the break-through step is carried out to an "over etch"
condition, so that not only is the silicon oxide layer 404 removed, but a
portion of the upper surface 414 of the silicon substrate 402 is etched in
a manner which generates a rounded shape 412 adjacent the sidewalls of the
silicon oxide layer 404. Typically, in order to obtain the rounded shape
412 near the surface 414 of the silicon substrate 402, it is only
necessary to etch the silicon substrate to a depth within the range of
about 100 .ANG. and about 400 .ANG., preferably, within the range of about
100 .ANG. and about 200 .ANG., during the silicon oxide breakthrough etch.
The breakthrough etch is performed using a specialized plasma generated
from a plasma feed gas comprising carbon and fluorine. The reactive
species generated from the carbon and fluorine-comprising plasma react
with the silicon oxide in layer 404 to form the volatile reaction products
SiF.sub.4 and CO.sub.2. The availability of carbon from residual
photoresist layer 408, and from the plasma feed gas make possible the
silicon oxide etching at a lower substrate bias (typically less than about
--200 V), which provides a gentle rounding of the upper trench corners,
since these corners are not as highly bombarded by ions attracted by the
substrate bias. In instances where there is minimal photoresist residue
available, it may be necessary to increase the amount of carbon present in
the plasma feed gas. This may be done by increasing the amount of a
carbon-fluorine containing compound in the feed gas, by addition of a
carbon-containing compound such as CH.sub.4 to the feed gas, or a
combination of both.
The break-through etch step is carried out for a time period sufficient to
provide an overetch. (For a feature size of about 0.35 .mu.m, a typical
overetch is about 300-400 .ANG. into the silicon substrate and the
rounding of the upper corners of the silicon trench provides a rounded top
corner having a radius of about 15 nm to about 25 nm). The trench is
subsequently etched to a desired depth using a different etch chemistry.
It is also possible that there is some formation of polymeric residue near
the sidewall of the etched silicon oxide layer 404 which acts as a
sacrificial mask for the silicon substrate directly underneath the silicon
oxide layer, and that this contributes to the rounding of the upper corner
of the silicon surface during the break-through step.
The plasma feed gas for the break-through step may further include oxygen
(O.sub.2), which may be used to improve the top corner rounding effect.
The presence of O.sub.2 during the overetch of the silicon results in the
production of silicon oxide, which may serve as a passivating agent. at
the edge of the top trench corner. The presence of the polymeric residue
adjacent the sidewall of the etched silicon oxide layer, in combination
with a silicon oxide build-up in the same area during the overetch portion
of the break-through step, may contribute to formation of the rounded top
corner 412 at the upper surface 414 of the silicon substrate 402 during
subsequent etching of the silicon trench.
The principal etchant of the plasma feed gas which is used to perform the
breakthrough etch may be selected from the group consisting of CF.sub.4,
CHF.sub.3, CH.sub.2 F.sub.2, CH.sub.3 F, and combinations thereof.
Experimental results have demonstrated that a principal etchant selected
from the group consisting of CF.sub.4, CHF.sub.3, and combinations thereof
works well. CF.sub.4 provides excellent results. If there is insufficient
photoresist residue available, or the imaging mask used for initial
patterning of the etch stack does not contain carbon, the plasma feed gas
may further include CH.sub.4 to increase the amount of polymer formation.
Further, the addition of O.sub.2 may be used to adjust the amount of
polymer formation when CH.sub.4 is added to the plasma source gas.
The plasma feed gas for the breakthrough etch preferably further includes a
nonreactive, diluent gas selected from the group consisting of argon,
helium, xenon, krypton, and combinations thereof. The nonreactive, diluent
gas is most preferably argon. The nonreactive, diluent gas dilutes the
carbon and fluorine-containing gas, slowing down the overall rate of
etching and increasing the selectivity of etching the silicon oxide layer
relative to etching of the photoresist layer. The term "selectivity"
refers either to 1) the ratio of the etch rate of a first material to the
etch rate of a second material, or 2) the etch rate of a first material
divided by the etch rate of a second material. If the first material has a
faster rate of etching than the second material, then selectivity will be
greater than 1:1. In general, a fast silicon oxide etch rate is not
desired, because the silicon oxide layer is so thin that etching is
completed quickly even if the silicon oxide etch rate is relatively low,
and it is desired to provide time to permit formation of the polymer-oxide
combination build-up adjacent the sidewall of the etched silicon oxide
layer during the silicon oxide breakthrough etch step.
Table One, below, provides process conditions for the first method of top
corner rounding described above. In addition to providing the process
conditions for the break-through, corner rounding step, the table provides
process conditions for a compatible pattern etching of the silicon nitride
layer. This compatible silicon nitride pattern etching step can be carried
out in the same process chamber as the break-through, corner rounding
step. The process conditions provided in Table One are specific to the use
of a processing apparatus in which the source power and bias power are
separately controlled, such as the Applied Materials' CENTURA.RTM. DPS.TM.
polysilicon etch system. However, the first top corner rounding method is
not intended to be limited to the use of this particular apparatus. One
skilled in the art can use the teachings provided herein to adapt the
corner rounding method to other apparatus.
TABLE ONE
Example Process Conditions for Silicon Nitride Pattern Etching
and the First Method of Silicon Trench Top Corner Rounding
Silicon Oxide
Silicon Nitride Breakthrough
Process Parameter Pattern Etching Etch
CF.sub.4 Flow Rate (sccm) -- 20-140
CH.sub.4 Flow Rate (sccm) -- 0-40
SF.sub.6 Flow Rate (sccm) 30-120 --
HBr Flow Rate (sccm) 30--160 --
O.sub.2 Flow Rate (sccm) -- 0-20
Argon Flow Rate (sccm) -- 40-160
Typical Total Gas Flow (sccm) 60-280 60-200
RF Source Power (W) 300-1200 300-1200
RF Bias Power (W) 50-450 25-150
Process Chamber Pressure (mTorr) 10-80 10-80
Substrate Temperature (.degree. C.) 10-70 10-70
Plasma Electron Density (e.sup.- /cm.sup.3) .sup. 10.sup.9 -10.sup.12
.sup. 10.sup.9 -10.sup.12
After performance of the silicon oxide break-through step, the general
etching of the trench in the silicon substrate is performed. FIG. 4D shows
the structure 400 of FIG. 4C after etching of a shallow trench 416 into
the silicon substrate 402. The trench is typically etched to a depth
within the range of about 2000 .ANG. to about 6000 .ANG.. The final trench
depth is dictated by the final end use application for the trench and the
electrical requirements for the resulting semiconductor device.
The trench 416 is etched using conventional silicon etch chemistry. The
plasma feed gas for the silicon trench etch preferably comprises HBr in
combination with O.sub.2. Optionally, Cl.sub.2 and/or SF.sub.6 may be
added to the plasma feed gas.
EXAMPLE ONE
The semiconductor structure 400 shown in FIG. 4 was the starting substrate.
This structure included a patterned photoresist layer 408, a silicon
nitride layer 406, and a silicon oxide layer 404, deposited on a
single-crystal silicon substrate 402. The patterned photoresist 408 was a
Deep UV (DUV) photoresist proprietary to and provided by Shipley Co.
(Massachusetts). The thickness of patterned photoresist 408 was
approximately 6000 .ANG.. The silicon nitride layer 406 had a thickness of
approximately 1500 .ANG.. The silicon oxide layer 404 had a thickness of
approximately 120 .ANG..
The silicon nitride layer 406 was pattern etched using a plasma generated
from a feed gas of 60 sccm SF.sub.6 and 60 sccm HBr. The silicon nitride
layer 406 was etched using the following process conditions: 40 mTorr
process chamber pressure; 300-1500 W plasma source power (depending on the
desired etch rate); 150 W bias power; and a substrate temperature of about
20.degree. C. Etching was stopped upon reaching the silicon
nitride/silicon oxide interface. The silicon nitride/silicon oxide
interface was detected by optical emission spectroscopy (OES).
The first top corner rounding method of the present invention was then
performed during the break-through step in which the silicon oxide layer
and a portion of the silicon substrate were etched. The silicon substrate
was etched to a depth of approximately 200 .ANG..
Various etchant gas compositions and process conditions were used to
perform the silicon oxide breakthrough step. Tables Two through Five,
below, provide process conditions such as plasma feed gas composition,
process chamber conditions, and power input parameters for a number of
experimental trials which were carried out during the development of the
first top corner rounding method.
TABLE TWO
First Top Corner Rounding Method, Developmental Data
Run # 1 2 3 4 5 6 7
CF.sub.4 (sccm) -- -- -- 80 50 80 80
SF.sub.6 (sccm) 20 20 20 -- -- -- --
Ar (sccm) -- -- -- -- -- 120 120
HBr (sccm) 140 140 140 -- -- -- --
O.sub.2 (sccm) -- 10 10 -- 5 -- --
He (sccm) -- -- -- -- -- -- --
He--O.sub.2 (sccm) -- -- -- -- -- -- --
Total Gas Flow 160 170 170 80 55 200 200
(sccm)
Proc. Chamber 5 5 80 70 4 70 10
Pressure (mTorr)
Source Power (W) 750 750 750 1000 500 1000 1000
Bias Power (W) 100 100 100 70 40 100 100
Substrate Temp. (.degree. C.) 50 50 50 50 50 50 50
SiO.sub.2 Etch Time (s) 40 40 40 40 40 30 15
Si Etch Time (s) 80 80 80 80 60 60 60
Si Etch Rate 2885 3078 1121 910 919 852 1392
(.ANG./min)
Si Etch Uniformity* 2.2 2.9 5.4 3.0 2.0 2.9 1.8
SiO.sub.2 :Si (Selectivity) 0.27 0.24 0.10 0.36 1.0 0.72 1.6
With regard to the above table and subsequent tables, the following
applies:
*Etch uniformity was measured on patterned, undoped polysilicon substrate
specimens, using a Tencor UV 1020 thin film thickness measurement tool. An
acceptable etch uniformity of about 3% or less variation was obtained.
TABLE THREE
First Top Corner Rounding Method, Developmental Data
Run # 8 9 10 11 12 13 14
CF.sub.4 (sccm) 80 80 80 80 80 80 40
SF.sub.6 (sccm) -- -- -- -- -- -- --
Ar (sccm) 120 -- 120 120 120 120 60
HBr (sccm) -- -- -- -- -- -- --
O.sub.2 (sccm) -- -- 10 20 -- -- --
He (sccm) -- -- -- -- -- -- --
He--O.sub.2 (sccm) -- -- -- -- -- -- --
Total Gas Flow 200 80 210 220 200 200 100
(sccm)
Proc. Chamber 10 10 10 10 10 10 10
Pressure (mTorr)
Source Power (W) 1000 1000 1000 1000 1000 300 1000
Bias Power (W) 100 100 100 100 200 100 100
Substrate Temp. (.degree. C.) 20 20 20 20 20 20 20
SiO.sub.2 Etch Time (s) 15 15 15 15 15 20 15
Si Etch Time (s) 40 40 40 40 40 40 40
Si Etch Rate 1405 1482 1684 1883 2102 1007 1296
(.ANG./min)
Si Etch Uniformity* 1.1 2.2 1.4 1.8 0.69 2.2 0.83
SiO.sub.2 :Si (Selectivity) 1.6 1.4 1.3 1.1 1.4 0.98 1.6
TABLE FOUR
First Top Corner Rounding Method, Developmental Data
Run # 15 16 17 18 19 20 21
CF.sub.4 (sccm) 80 80 20 20 10 10 --
SF.sub.6 (sccm) -- -- -- -- -- -- --
Ar (sccm) 120 -- 60 120 120 200 120
HBr (sccm) -- -- -- -- -- -- --
O.sub.2 (sccm) -- 20 -- -- -- -- --
He (sccm) -- -- -- -- -- -- --
He--O.sub.2 (sccm) -- -- -- -- -- -- --
Total Gas Flow 200 100 80 140 130 210 120
(sccm)
Proc. Chamber 30 10 30 30 30 30 30
Pressure (mTorr)
Source Power (W) 1000 1000 1200 1200 1200 1200 1200
Bias Power (W) 100 100 50 50 50 50 50
Substrate Temp. (.degree. C.) 20 20 20 20 20 20 20
SiO.sub.2 Etch Time (s) 15 15 30 30 30 30 30
Si Etch Time (s) 40 40 40 40 40 40 40
Si Etch Rate 1179 1887 815 616 420 388 60
(.ANG./min)
Si Etch Uniformity* 2.8 2.3 2.2 3.6 2.9 3.4 5.2
SiO.sub.2 :Si (Selectivity) 1.3 1.1 1.6 2.0 2.4 1.2 1.1
TABLE FIVE
First Top Corner Rounding Method, Developmental Data
Run # 22 23 24 25 26 27
CF.sub.4 (sccm) 10 10 10 40 40 40
SF.sub.6 (sccm) -- -- -- -- -- --
Ar (sccm) 200 -- 200 120 120 120
HBr (sccm) 20 20 40 -- -- --
O.sub.2 (sccm) -- -- -- -- -- --
He (sccm) 10 -- 20 -- -- --
He--O.sub.2 (sccm) -- 10 -- -- -- --
Total Gas Flow 240 40 270 160 160 160
(sccm)
Proc. Chamber 30 30 30 30 10 30
Pressure (mTorr)
Source Power (W) 1200 1200 1200 750 750 750
Bias Power (W) 50 50 50 50 50 100
Substrate Temp. (.degree. C.) 20 20 20 20 20 20
SiO.sub.2 Etch Time (s) 30 20 30 30 20 20
Si Etch Time (s) 40 40 40 40 40 40
Si Etch Rate 1520 1606 1410 700 653 1030
(.ANG./min)
Si Etch Uniformity* 9.1 2.4 8.0 1.8 1.5 1.9
SiO.sub.2 :Si (Selectivity) 0.44 0.34 0.41 1.3 1.8 1.3
Silicon oxide: silicon (SiO.sub.2 : Si) etch selectivity is preferably
greater than 1. The maximum SiO.sub.2 : Si selectivity (2.4) was obtained
in Run #19 (refer to Table Four), using the following process conditions:
10 sccm CF.sub.4, 120 sccm argon, 30 mTorr process chamber pressure, 1200
W source power, 50 W bias power, and a 20.degree. C. substrate
temperature. The optimum top trench corner rounding effect was observed in
Run #25 (refer to Table Five), where the process conditions were the same
as those in Run #19, with the following exceptions: 40 sccm CF.sub.4 and
750 W source power. A typical etch time for about 100 .ANG. of oxide was
about 20 seconds. For a given etch process, one skilled in the art can
adjust the relative amount of the carbon and fluorine comprising compounds
to the plasma feed gas and the amount of power applied to plasma
generation to obtain a satisfactory top trench corner rounding effect,
while providing an advantageous silicon oxide: silicon etch selectivity.
III. A SECOND METHOD OF ETCHING A SILICON TRENCH HAVING ROUNDED TOP CORNERS
FIG. 5A shows a schematic of a semiconductor structure 500 comprising an
etch stack overlying a silicon substrate 502. This etch stack comprises,
from top to bottom, a patterned silicon nitride hard mask 506 and a
silicon oxide layer 504 overlying a silicon substrate 502. The silicon
nitride hard masking layer 506 typically has a thickness within the range
of about 1000 .ANG. to about 2000 .ANG.. The silicon oxide layer 504
typically has a thickness within the range of about 50 .ANG. to about 350
.ANG..
The break-through step used to etch through the silicon oxide layer 504 as
shown in FIG. 5B is also used to create a built-up extension 508 upon
sidewall 507 of silicon nitride hard mask 506. The second top corner
rounding method break-through step is performed using a plasma generated
from a plasma feed gas comprising a source of hydrogen, a source of
carbon, a source of fluorine, and a source of surface bombardment atoms.
Typically, the surface bombardment atom source is an inert plasma feed gas
such as argon, helium, krypton, nitrogen, xenon, or a combination thereof.
Argon works particularly well. A single compound may be used to provide
the hydrogen, carbon, and fluorine. Typically, more than one gaseous
compound is used. It is important to have a controlled amount of carbon
present in the process chamber at the time of the break-through step.
While the hydrogen and the carbon are used to form polymeric material
which deposits upon the silicon nitride sidewall, the fluorine and
bombardment atoms (typically argon) provide for removal of native oxide
layers. Examples of hydrogen sources include H.sub.2, HBr (which requires
the presence of an increased amount of carbon in the process chamber to
work well), NH.sub.3, CHF.sub.3, CH.sub.2 F.sub.2, CH.sub.3 F, CH.sub.4,
and combinations thereof. Several of these hydrogen-containing compounds
may also serve as carbon and fluorine sources. An excellent fluorine
source is CF.sub.4.
One of the recommended plasma feed gas recipes is a combination of
CF.sub.4, HBr, and argon. The combination of the CF.sub.4 with either HBr,
CH.sub.2 F.sub.2, or a combination of these gases results in the formation
of a C.sub.x H.sub.y -based polymer on the silicon nitride sidewall 507.
This combination of plasma feed gases is capable of providing polymer
deposits in the absence of a photoresist, and is not dependent on the
amount of residue available from a previous process step.
The plasma feed gas chemistry (i e., CF.sub.4, argon, and HBr or CH.sub.2
F.sub.2) is preferably made more reactive using an RF source capable of
producing a higher electron density. The combination of polymer deposits
with byproducts from the silicon oxide etch which make up extension 508
act as a sacrificial mask to prevent the silicon at the silicon
oxide/silicon interface at the top of the trench from being etched until
desired.
A combination of a high process chamber pressure (within the range of about
40 mTorr to about 90 mTorr) and a low substrate bias (within the range of
about -300 V to about -350 V) during the silicon oxide breakthrough step
enhances the chemical reaction of the gases and prevents directional
etching of the silicon substrate. The depth to which the silicon 502 is
etched during the breakthrough step is minimal (typically less than about
100 .ANG. during a 20-second breakthrough etch), so silicon erosion
adjacent silicon oxide layer 504 is not a problem. Argon gas may be used
to enhance the plasma density and to provide bombarding high energy
species which assist in break through of native oxides.
Because the extension 508 deflects the etchant species further down the
substrate sidewall, the result is the formation of a more rounded shape (a
corner or edge 512 having a larger radius) on the upper sidewall of the
silicon substrate.
Table Six, below, provides process conditions which have shown excellent
results for performlng a silicon nitride pattern etching step which is
compatible with the silicon oxide break-through step, and for the silicon
oxide breakthrough step in which an extension is formed on the side walls
of the silicon nitride hard mask. The process conditions provided in Table
Six are specific to the use of a processing apparatus in which the source
power and bias power are separately controlled, such as the Applied
Materials' CENTURA.RTM. DPS.TM. polysilicon etch system.
TABLE SIX
Process Conditions for Silicon Nitride Pattern Etching
and the Second Method for Silicon Trench Top Corner Rounding
Silicon Nitride Silicon Oxide
Process Parameter Pattern Etching Break-through Step
CF.sub.4 Flow Rate (sccm) -- 20-80
SF.sub.6 Flow Rate (sccm) 30-120 --
HBr Flow Rate (sccm) 30-160 50-100*
CH.sub.2 F.sub.2 (sccm) -- 30-150*
O.sub.2 Flow Rate (sccm) -- --
Argon Flow Rate (sccm) -- 60-120
Typical Total Gas Flow (sccm) 60-280 110-300
RF Source Power (W) 300-1200 1000-1800
RF Bias Power (W) 50-450 40-100
Substrate Bias Voltage (-V) -200--450
Process Chamber Pressure (mTorr) 10-80 50-90
Substrate Temperature (.degree. C.) 10-70 10-50
Plasma Electron Density (e.sup.- /cm.sup.3) .sup. 10.sup.9 -10.sup.12
.sup. 10.sup.9 -10.sup.12
*Generally, either HBr orCH.sub.2 F.sub.2 is used. It is possible to use a
combination of such hydrogen source gasses, but this is a complication of
the etch process.
Specific examples for performing the method of the present invention are
provided below.
EXAMPLE TWO
The semiconductor structure 500 shown in FIG. 5A was the starting substrate
for the second top corner rounding method. This structure includes a
silicon nitride hard mask 506 and silicon oxide layer 504, deposited on a
single-crystal silicon substrate 502, as previously described. The silicon
nitride masking layer 506 had a thickness of approximately 2,000 .ANG..
The silicon oxide layer 504 had a thickness of approximately 150 .ANG..
The silicon oxide break-through step which also formed extension 508 was
performed using a plasma generated from a plasma feed gas containing 40
sccm CF.sub.4, 75 sccm argon, and either 50 sccm CH.sub.2 F.sub.2 or 70
sccm HBr. The silicon oxide breakthrough step was performed using the
following process conditions: 70 mTorr process chamber pressure; 1200 W
source power; 70 W (-335 V) bias power; and 20.degree. C. substrate
temperature.
FIG. 5B shows the structure 500 of FIG. 5A following the silicon oxide
breakthrough step. There was a critical dimension gain of approximately
150 .ANG., which represents the extension 508 formed on the sidewall 507
of the silicon nitride layer 506.
Silicon trench etching was then performed using the two-step silicon etch
process described in Section VI, below, which provides rounded top corners
and rounded bottom corners. This two-step silicon trench etch is useful
not only as a part of this second method for etching a silicon trench
having rounded top corners, but can also be used in combination with the
first method for etching a silicon trench having rounded top corners.
IV. A THIRD METHOD OF ETCHING A SILICON TRENCH HAVING ROUNDED TOP CORNERS
FIG. 8A shows a schematic of a semiconductor structure 800 comprising an
etch stack overlying a silicon substrate 802. This etch stack comprises,
from top to bottom, a patterned photoresist layer 808, a silicon nitride
layer 806, and a silicon oxide layer 804. The thickness of the patterned
photoresist layer 808 is typically within the range of about 4,000 .ANG.
to about 10,000 .ANG.. The silicon nitride layer 806 typically has a
thickness within the range of about 1,000 .ANG. to about 2,000 .ANG.. The
silicon oxide layer 804 typically has a thickness within the range of
about 50 .ANG. to about 350 .ANG.. The relative thicknesses of the film
layers illustrated in FIGS. 8A-8D are not to scale. The drawings are
intended only to show the relative positions of the various layers on
silicon substrate 802.
The patterned photoresist layer 808 may be any suitable photoresist
material patterned using techniques known in the art. Typically, the
photoresist layer material is an organic, carbon-containing material. The
thickness and patterning method for the photoresist 808 will depend on the
particular photoresist material used. A frequently used photoresist is a
DUV photoresist available from either JSR or Shipley, and the thickness
for such a photoresist layer is the about 4,000 to about 10,000 .ANG.
range provided above.
Referring to FIG. 8B, the silicon nitride layer 806 is pattern etched
(opened) to expose a surface 810 of the silicon oxide layer 804.
Preferably, the silicon nitride layer 806 is pattern etched using a plasma
generated from a feed gas which does not result in polymer deposition on
surfaces of the semiconductor structure during etching. The feed gas
preferably does not contain carbon. The feed gas used during plasma
etching of the silicon nitride layer 806 typically comprises SF.sub.6 or
CF.sub.4 or a combination thereof, used in combination with HBr. Etch
process conditions are apparatus-sensitive, but one skilled in the art can
determine appropriate conditions for a given apparatus by minimal
experimentation, based on process conditions generally known and published
in the art.
Referring to FIG. 8C, the silicon oxide layer 804 is then etched to expose
a surface 811 of the silicon substrate 802. To prevent deposition of
polymer on surfaces of the semiconductor structure during etching, the
silicon oxide layer is plasma etched using a feed gas which does not
result in polymer generation. Preferably, the silicon oxide layer 804 is
plasma etched using a feed gas which does not contain hydrogen. One feed
gas commonly used during plasma etching of the silicon oxide layer 804
comprises CF.sub.4 and argon. Etch process conditions are
apparatus-sensitive, but one skilled in the art can determine appropriate
conditions for a given apparatus by minimal experimentation, based on
process conditions generally known and published in the art.
Although FIG. 8C does not show any overetch of silicon oxide layer 804 into
underlying silicon substrate 802, in some instances, this is helpful in
obtaining control over the critical dimension of the patterned silicon
oxide layer 804.
FIG. 8D shows the structure 800 of FIG. 8C after the top corner rounding
step. During the top corner rounding step, the silicon substrate is plasma
etched for a short period of time (typically about 10-30 seconds, which is
sufficient to provide an etch depth of about 50 to about 200 .ANG.) using
a feed gas comprising a source of fluorine, a source of carbon, a source
of hydrogen, and a source of high energy species which provide physical
bombardment of the silicon substrate. The carbon and hydrogen within the
feed gas combine to provide deposition of a thin layer of polymer on the
top corner of the trench during etching of the first portion of the
trench, resulting in the formation of a rounded "shoulder" 812 at the top
trench corner.
Preferred feed gases for use in the top corner rounding step are CHF.sub.3
/HBr/CF.sub.4 or CHF.sub.3 /Ar/CF.sub.4. CH.sub.2 F.sub.2 may be used in
place of CHF.sub.3, although the use of CH.sub.2 F.sub.2 generally
produces an increase in the amount of polymer deposition and may require
an adjustment in the relative amounts of the plasma feed gas components.
Since some of the top corner rounding polymer deposition is removed during
etch of the remainder of the silicon trench, it may be preferable to use
CH.sub.2 F.sub.2 for top corner rounding when the silicon trench is
deeper. Krypton, xenon, helium, and combinations thereof may be used in
place of HBr or argon to provide physical bombardment of the substrate.
If, for example, helium is used in place of argon, a substantially greater
feed rate of helium must be used to obtain equivalent physical bombardment
to argon, due to the much smaller size and weight of the helium atom.
For a shallow trench having a final depth within the range of about 2000
.ANG. to about 6000 .ANG., the top corner rounding step is typically
performed for a time period within the range of about 10 seconds to about
30 seconds. For deeper trenches (5000-6000 .ANG.), the top corner rounding
time should be toward the high end of this time range in order to allow
for sufficient polymer deposition at the top corners of the trench. For a
given trench depth, the optimum amount of time required will vary
depending on the particular equipment and process conditions used, and can
easily be determined through routine experimentation. Etching of the
silicon substrate during the top corner rounding step is typically
performed to a depth of about 50 .ANG. to about 200 .ANG., depending on
the desired final trench depth.
Table Seven, below, provides typical process conditions for the third
method of top corner rounding described above. In addition to providing
the process conditions for the silicon oxide etch and top corner rounding
steps, the table provides process conditions for a compatible pattern
etching of the silicon nitride layer. This compatible silicon nitride
pattern etching step can be carried out in the same process chamber as the
silicon oxide etch and top corner rounding steps. The process conditions
provided in Table Seven are specific to the use of a processing apparatus
in which the source power and bias power are separately controlled, such
as the Applied Materials' CENTURA.RTM. DPS.TM. polysilicon etch system, of
the kind shown in FIGS. 1 and 2. However, the third top corner rounding
method is not intended to be limited to use of this specific apparatus.
One skilled in the art can use the teachings provided herein to adapt the
corner rounding method to other apparatus.
TABLE SEVEN
Example Process Conditions for
Silicon Nitride Pattern Etching, Silicon Oxide
Etching and the Third Method of Silicon Trench Top Corner Rounding
Silicon Top
Silicon Nitride Oxide Corner
Process Parameter Pattern Etching Etch Rounding
CF.sub.4 Flow Rate (sccm) -- 40-100 10-50
CHF.sub.3 Flow Rate (sccm) -- -- 40-100
SF.sub.6 Flow Rate (sccm) 30-120 -- --
HBr Flow Rate (sccm) 30-160 -- 10-100
Argon Flow Rate (sccm) -- 50-150 50-150
Typical Total Gas Flow 60-280 100-300 110-300
(sccm)
RF Source Power (W) 300-1200 700-1300 700-1800
RF Bias Power (W) 50-450 40-150 40-300
Process Chamber Pressure 10-80 4-90 40-90
(mTorr)
Substrate Temperature 10-70 10-70 10-60
(.degree. C.)
Plasma Electron .sup. 10.sup.9 -10.sup.12 .sup. 10.sup.9
-10.sup.12 .sup. 10.sup.9 -10.sup.12
Density
(e.sup.- /cm.sup.3)
After performance of the top corner rounding step, the feed gas may be
adjusted to an etch chemistry such as HBr/Cl.sub.2 /O.sub.2 or Cl.sub.2
/O.sub.2, or HBr, or HBr/Cl.sub.2, for example, a by way of limitation.
FIG. 8E shows the structure 800 of FIG. 8D after etching of a shallow
trench 816 into the silicon substrate 802. The trench is typically etched
to a depth within the range of about 2000 .ANG. to about 6000 .ANG.. The
final trench depth is dictated by the final end use application of the
trench and the electrical requirements for the resulting semiconductor
device.
Hydrogen-based polymers deposited on upper surfaces of the semiconductor
structure will typically be removed during an oxide-based silicon trench
etch. However, the polymer "shoulders" generated at the top corners of the
trench during the trench corner rounding step are typically protected by
the shadowing effect of the silicon nitride hard mask during etching of
the trench into the silicon substrate. However, if silicon trench etching
proceeds for a sufficient time period, the corner-rounding polymer coating
may be removed. In the embodiment described above, etching of a silicon
trench about 7000 .ANG. in depth having rounded top corners was
successfully achieved.
After etching of the trench in the silicon substrate, a cleaning step is
performed using an O.sub.2 plasma if necessary to remove any remaining
photoresist. Silicon oxide polymers generated during the silicon trench
etch and subsequent cleaning step may be removed by performing a wet
cleaning step. Wet cleaning is typically performed using either a hydrogen
fluoride (HF) dip or EKC.TM. wafer cleaning chemicals available from EKC
Technology, Inc. of Hayward, Calif. Although the HF dip is very effective
at removing oxide-based polymers, it is not particularly effective at
removing hydrogen-based polymers which may remain on surfaces of the
semiconductor structure following silicon trench etch. Therefore, it is
important to minimize the amount of hydrogen-based polymer deposited
during performance of the etch method of the invention.
FIGS. 9A through 9C show schematics of a structure 900 after a top corner
rounding step. FIG. 9A shows a 900 comprising, from top to bottom, a
patterned masking layer of silicon nitride 906, an etched layer of silicon
oxide 904, and an underlying silicon substrate 902. FIG. 9B shows an
enlargement of the rounded corner achieved using an embodiment of the
method of the present invention, where the silicon oxide layer 904 is not
overetched into the silicon substrate prior to the top corner rounding
step. FIG. 9C shows an enlargement of the rounded corner achieved using an
embodiment of the method where the silicon oxide layer is overetched into
the silicon substrate prior to the top corner rounding step, to provide a
gauged top rounded corner.
V. A METHOD OF ETCHING A SILICON TRENCH HAVING ROUNDED BOTTOM CORNERS
FIGS. 5C and 5D illustrate a two-step silicon trench etch process, wherein
the first step provides a rounded top corner in the second method of
etching a silicon trench having rounded top corners, and wherein the
second step provides a rounded bottom corner, if desired. The two step
silicon trench etch process, when used in conjunction with either the
first or second method for providing top corner rounding, produces overall
curved trench profiles, with minimal risk of trench stress areas.
In the first silicon etch step, shown in FIG. 5C, a trench 518 is etched to
a depth "A", which is within the range of about 75% and about 95% of its
desired final depth, using conventional silicon etch chemistry with a
process chamber pressure within the range of about 20 mTorr and about 40
mTorr. For example and not by way of limitation, the first silicon etch
step is typically performed using a plasma generated from a plasma feed
gas comprising Cl.sub.2 and O.sub.2, under the following process
conditions: 90 sccm Cl.sub.2 ; 10 sccm O.sub.2 ; 30 m Torr chamber
pressure; 1,000 W source power; and 200 W bias power. Other silicon etch
chemistry known in the art may be used as well. The important concept is
that only a portion of the trench is etched at the initial chamber
pressure, with the subsequent completion of the trench etch being carried
out at increased chamber pressure.
In the second silicon etch step, shown in FIG. 5D, the process chamber
pressure is increased. The trench 518 is etched to its desired final depth
B using conventional silicon etch chemistry with a process chamber
pressure within the range of about 50 mTorr and about 90 mTorr, resulting
in rounding of the bottom trench corners 520. For example and not by way
of limitation, the second silicon etch step is typically performed using a
plasma generated from a plasma feed gas comprising Cl.sub.2, O.sub.2, and
SF.sub.6, under the following process conditions: 90 sccm Cl.sub.2 ; 10
sccm O.sub.2 ; 10 sccm SF.sub.6 ; 55 mTorr chamber pressure; 1,000 W
source power; and 200 W bias power. SF.sub.6 is optionally added to the
plasma feed gas in the second silicon etch step for the purpose of
enhancing lateral etching, to provide better bottom corner rounding.
Table Eight, below, provides general process conditions for each step of
the two-step silicon trench etch process. The process conditions provided
in Table Eight are specific to the use of a processing apparatus in which
the source power and bias power are separately controlled, such as the
Applied Materials' CENTURA.RTM. DPS.TM. polysilicon etch system.
TABLE EIGHT
Process Conditions for Two-Step Silicon Trench Etch
Silicon Trench Silicon Trench
Process Parameter Etch First Step Etch Second Step
SF.sub.6 Flow Rate (sccm) -- 5-20
O.sub.2 Flow Rate (sccm) 2-20 2-20
Cl.sub.2 Flow Rate (sccm) 40-200 40-250
Typical Total Gas Flow (sccm) 42-220 42-270
RF Source Power (W) 800-1600 800-1600
RF Bias Power (W) 150-400 150-400
Substrate Bias Voltage (-V) 400-600 400-600
Process Chamber Pressure (mTorr) 10-50 50-100
Substrate Temperature (.degree. C.) 10-70 10-70
Plasma Electron Density (e.sup.- /cm.sup.3) .sup. 10.sup.9 -10.sup.12
.sup. 10.sup.9 -10.sup.12
EXAMPLE THREE
A two-step silicon etch process was then performed to provide a trench with
a rounded bottom. In the first silicon etch step, the trench was etched to
a depth of 2,500 .ANG. using 90 sccm Cl.sub.2 and 5 sccm O.sub.2. The
first silicon etch step was performed using the following process
conditions: 25 mTorr process chamber pressure; 1000 W source power; 200 W
(-500 V) bias power; and 20.degree. C. substrate temperature.
In the second silicon etch step, the trench was etched to a final depth of
3,000 .ANG. using 90 sccm Cl.sub.2, 5 sccm O.sub.2, and 10 sccm SF.sub.6.
The second silicon etch step was performed using the following process
conditions: 55 mTorr process chamber pressure; 1000 W source power; 200 W
(-520 V) bias power; and 20.degree. C. substrate temperature. The
increased process chamber pressure, in combination with the presence of
SF.sub.6, during the second silicon etch step enhanced the lateral
etching, resulting in rounding of the bottom trench corners.
Using prior art methods, the silicon trench etch step typically has a
somewhat deleterious effect on the silicon nitride taper angle. However,
when the silicon oxide breakthrough chemistry of the present invention is
used, the silicon nitride taper angle is not adversely affected.
Preliminary experiments showed that the etch depth uniformity across the
wafer is well within industry requirements using this breakthrough
chemistry. Etch rate microloading was measured to be less than about 2%.
The methods of the present invention provide for plasma etching a trench
having rounded top and bottom corners in a silicon substrate. Using this
method in combination with known etch techniques for controlling the
silicon trench sidewall taper, it is possible to provide a predetermined
cross-sectional trench sidewall shape, as illustrated in FIGS. 6A through
6F. The term "etch profile", "trench profile", or "feature profile"
generally refers to, but is not limited to, the cross-sectional profile of
the sidewall of an etched feature. In many instances herein, the etch
profile is described in terms of an angle between the sidewall and the
surface on which the feature stands (i.e., the substrate). The term
"vertical profile" refers to a feature profile wherein a cross-section of
the feature exhibits sidewalls which are perpendicular to the surface on
which the feature stands. The term "positive profile" (also known as an
"undercut" profile) refers to a feature profile wherein the width of the
cross-section of the feature is larger as the distance away from the
opening on the substrate increases. The term "negative profile" (also
known as a "tapered" profile) refers to a feature profile wherein the
width of the cross-section of the feature is smaller as the distance away
from the opening on the substrate surface increases.
In FIG. 6A, the silicon trench profile 602 exhibits a top corner radius 603
and a bottom corner radius 605 which are tighter (smaller), and the slope
of sidewall 604 relative to the base 606 of profile 602, as indicated by
angle .theta. 607, is nearly 90 degrees. In FIG. 6B, the silicon trench
profile 612 exhibits a top corner radius 613 and a bottom corner radius
615 which are more open (larger), while the slope of sidewall 614 relative
to the base 616 of profile 612, as indicated by angle .theta. 617, remains
at nearly 90 degrees. An increase in top corner radius may be achieved by
using the second top corner rounding method rather than the first top
corner rounding method, as described above. Within a given corner rounding
method (top or bottom corner rounding), an increase in radius is generally
achieved by increasing the etch time during corner formation.
In FIG. 6C, the silicon trench profile 622 exhibits a top corner radius 623
and a bottom corner radius 625 which are more open (larger). In addition,
the slope of sidewall 624 relative to the base 626 of profile 622, as
indicated by angle .theta. 627, has been increased to provide a tapered or
negative profile trench. In FIG. 6D, the silicon trench profile 632
exhibits a top corner radius 633 and a bottom corner radius 635 which are
more open (larger), while the slope of sidewall 634 relative to the base
636 of profile 632, as indicated by angle .theta. 637, provides a positive
profile trench. It is generally known in the art that the addition of HBr
to Cl.sub.2 /O.sub.2 etch chemistry will produce a more tapered (negative)
profile trench.
In FIGS. 6E and 6F, the silicon trench profiles 642 and 652, respectively,
exhibit a top corner radius (643 and 653) and a bottom corner radius (645
and 655) which are more open (larger), and the slope of the trench
sidewalls 644 and 654, respectively, provide a tapered or negative profile
trench. However, profile 642 is for a deeper trench than profile 652,
where the length of sidewall 644 of profile 642 is considerably greater
than the length of sidewall 654 of profile 652, so that the overall shape
of the trench profile is considerably different.
FIGS. 6A through 6F illustrate the considerable difference in the overall
shape of the trench profile which may be achieved by combining the corner
rounding methods of the present invention with methods for altering the
trench sidewall taper.
Preferably, the apparatus used to practice the present invention is adapted
to be controlled by a computer. FIG. 7 shows a computer 700. Computer 700
comprises a processor 702, memory 704 adapted to store instructions 706,
and one or more ports 708. Processor 702 is adapted to communicate with
memory 704 and to execute instructions 706. Processor 702 and memory 704
are also adapted to communicate with one or more ports 708. Ports 708 are
adapted to communicate with a plasma etch chamber 712. Plasma etch chamber
is adapted to carry out process steps in accordance with signals received
from processor 702 via ports 708. Preferably, computer 702 can control the
composition and feed rate of the plasma source gas, the temperature, the
pressure in the chamber, the bias power, the plasma source generation
power. Preferably, the plasma generation source and substrate biasing
device have separate power controls, allowing the plasma source power and
the substrate bias power to be controlled independently of one another.
Preferably, computer 702 is adapted to receive measurements that describe
the condition in the chamber, and adapt the process variables accordingly.
This programmed control of process variables enables production of a
predetermined device etch profile of the kind described above.
The above described preferred embodiments are not intended to limit the
scope of the present invention, as one skilled in the art can, in view of
the present disclosure, expand such embodiments to correspond with the
subject matter of the invention claimed below.
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